SAN FRANCISCO (CBS/AP) — Mark Zuckerberg and his wife, Priscilla Chan, were the most generous American philanthropists in 2013.

According to the Chronicle of Philanthropy’s annual list of the 50 top philanthropists, Zuckerberg and Chan topped the list with a total donation of 18 million shares of Facebook stock, valued at more than $970 million, to Silicon Valley nonprofits in December.

Zuckerberg’s gift of 18 million Facebook shares was valued at $498.8 million based on their closing stock price on the day of the gift, when Zuckerberg, 28, made the donation. The beneficiary is the Silicon Valley Community Foundation, a nonprofit that works with donors to allocate their gifts.

Zuckerberg also donated $5 million to help expand a community health center in Silicon Valley.

The gift covered part of the $29 million cost for adding another facility to the Ravenswood Family Health Center in East Palo Alto. That’s not far from Facebook Inc.’s headquarters in Menlo Park.

The top 50 donors made donations last year totaling $7.7 billion, plus pledges of $2.9 billion.

The Chronicle’s editor says the most significant fact from the list was the amount of money coming from living donors, which totaled about the same amount as the two previous years combined. She says that’s a sure sign the economy is getting better.